A practical starting point
How big a roof box for a family of four?
There is no single answer: four small soft bags can need less room than three large suitcases and a pushchair.
Try a family-of-four estimateCount the awkward things too
Cases, child seats, pushchairs, camping gear and sports equipment change the answer quickly. Leave spare capacity for soft bags that cannot be packed into neat cubes.
Check the roof box's maximum load, the bars and your vehicle handbook before loading.